i think the air popper is a stupid good starting device. i have not tried this, but i think adding these would give you crazy control for a simple electronics modification
you will probably need to keep your fan speed at max, because that's what stirs the beans around. so what you can vary, is the heat. so you'd open up your popper and you should be able to see the 120vac power cord powering the fan, and the heater. it probably goes through some switch on the front of the device or something.
instead of your heater getting power directly from the popcorn power switch, you have those wires go to this motor controller. hook it up to the "power" labeled connectors. then, on the controller box, have that hook up to the heater.
in terms of electronics, you're hooking up another switch in series with the heater. if you just want the heater to run at full power, you'd have the popcorn switch set to on, and this motor controller set to max.
these dang air poppers are a fluid air bed roaster, with the heater and fan ALMOST ready to be plugged into relays that should be able to control from an arduino or raspberry pi. their biggest downside is low batch size. but if you can roast and coal 100g batches every 8 minutes, it's not too slow.
while i'm working on the temp sensor, the part i'm stuck on is, i dont know what really strong fan and heater to buy for a larger fluid bed roaster. even then, what heat safe container do i put above the strong airflow? i saw someone else's fluid air bed roaster where they used parts from 3 french presses to make the roasting vessel. i think it would be safe, but it was much taller and spaced out than i would like.